Congenital muscular dystrophies
Most CMDs are autosomal recessive. This means that two copies of the defective gene, one from each parent, must be present in order for the disease to develop. If an individual inherits one defective gene and one normal gene, they will be a carrier, but not have the disease themselves. When both parents are carriers, they have a 25% chance with each pregnancy of having a child with the disease. They also have a 25% chance with each pregnancy of having an unaffected child. They have a 50% chance with each pregnancy of having a child who is also a carrier. The risk is the same for girls and boys.
Some CMDs, such as Collagen type VI diseases can be caused by either autosomal recessive, or autosomal dominant mutations. Autosomal dominant means that only a single copy of the defective gene must be present for the disease to develop. A parent with an autosomal dominant condition has a 50% chance with each pregnancy of having a child with the condition. Children who do not inherit the defective gene will not develop the disease or pass it on to their children.
The disease can also occur ‘de novo’, meaning that a child is born with the condition but neither parent has the affected gene.